Tunable excitation dynamics, fluorescence decay and microstructural analysis of Eu ₂ O ₃ , Nb ₂ O ₅ nanoparticles doped birefringent liquid crystals

In this investigation, liquid crystalline nanocomposite (LCNCs) of azobenzene derivatives<b>3a</b>(methyl 4-[(<i>E</i>)-(4-butoxy-2,5-difluorophenyl)diazenyl]-3-methoxybenzoate), and<b>3b</b>(methyl 4-{(<i>E</i>)-[2,5-difluoro-4-(hexyloxy)phenyl]diazenyl}-3-methoxybenzoate) were doped with 0.5 wt.% europium oxide (Eu₂O₃), niobium oxide (Nb₂O₅) nanoparticles (NPs) and named as<b>3a</b>@Eu₂O₃,<b>3b</b>@Nb₂O₅LCNCs. The powdered x-ray diffraction confirms the purity and integration of Eu₂O₃, Nb₂O₅NPs. Field emission scanning electron microscope studies demonstrate a non-uniform worm/flake-like morphology with agglomeration of LCNCs. A slight shift in optical band gap was observed for<b>3a, 3b</b>LCNCs, and broad absorption bands were observed in ultraviolet-visible studies. The existence of birefringent textures of<b>3a</b>@Eu₂O₃and<b>3b</b>@Nb₂O₅LCNCs is confirmed using a polarizing optical microscope. Room temperature photoluminescence (PL) studies demonstrate pronounced luminescence maxima with sharp emission peaks of violet, blue, green emissions under excitation at 380 nm, 460 nm, 360 nm, and 375 nm. Steady state PL (SSPL) of<b>3a</b>@Eu₂O₃LCNC exhibits luminescence maxima associated with vibrant red, blue, and greenish-yellow colors, while SSPL of<b>3b</b>@Nb₂O₅LCNC shows emission peaks of brilliant blue, yellow, green, orange, red colors at various<i>λ</i>_excitations. Furthermore, the large Stokes shift and full width at half maximum of<b>3a</b>@Eu₂O₃and<b>3b</b>@Nb₂O₅LCNCs are attributed to the presence of additional/virtual energy levels during the photoexcitation process. The fluorescence decay studies of<b>3a</b>@Eu₂O₃LCNC display average lifetime dwell between 19.8 ns and 0.276 ns, while<b>3b</b>@Nb₂O₅exhibits 9.69 ns to 10.20 ns, at various excitation wavelengths. The quantum yield varies between -7.77 × 10^-2% to 5.23 × 10^-2% and 0.86% to 0.14% for<b>3a</b>@Eu₂O₃and<b>3b</b>@Nb₂O₅LCNCs, respectively, upon increase in<i>λ</i>_excitations. Polarization studies of<b>3a</b>@Eu₂O₃LCNC at<i>λ</i>_{excitation/emission}angle (0°) showed intense defect-centered blue, red emission bands (445 nm, 613.8 nm) attributed to the excited state virtual orbitals that undergo non-radiative decay with higher energy content. Thus, the results obtained for synthesized<b>3a, 3b</b>LCs doped with Eu₂O₃and Nb₂O₅NPs make these usable for advanced optoelectronics, photonic, fluorescence/photo-luminescence device applications.